Catalysts for the preparation of methylpyridine

ABSTRACT

Subject of the invention is a dehydrogenation catalyst for dehydrogenating methylpiperidine to methylpyridine. Subject of the invention are also methods for preparing the catalysts obtained thereby and methods, in which the catalysts are used.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of priority from U.S. ProvisionalPatent Application No. 61/252,336 filed Oct. 16, 2009, the disclosure ofwhich is incorporated herein by reference.

Subject of the invention is a dehydrogenation catalyst fordehydrogenating methylpiperidine to methylpyridine. Subject of theinvention are also methods for preparing the catalysts and methods, inwhich the catalysts are used.

BACKGROUND OF THE INVENTION

3-methylpiperidine and 3-methylpyridine (3-picoline) are intermediatesin the industrial production of nicotinic amide and nicotinic acid,which is an essential vitamin of the B-complex (vitamin B₃). In thisprocess, 3-methylpiperidine is converted to 3-methylpyridine in thepresence of a dehydrogenation catalyst. The 3-methylpyridine isconverted to 3-cyanopyridine by oxidative ammonolysis. The3-methylpiperidine can be obtained by cyclization of2-methyl-1,5-diaminopentane.

Catalysts function to increase the rate of a chemical reaction at agiven temperature by lowering the necessary amount of energy to reachthe transition state. They can be present in the same phase as thereaction educts (homogenous catalysts) or in a different phase(heterogeneous catalysts).

Methylpyridines are also used as organic solvents. Further, they areused in organic synthesis for producing derivatized products thereof.3-picoline is a colourless, combustible liquid which is also used in theproduction of pharmaceuticals, dyes, rubber chemicals, resins andinsecticides.

EP 0 770 687 B1 discloses the industrial synthesis of nicotinic acidamides, starting from 2-methyl-1,5-diaminopentane. This compound isconverted to 3-methylpiperidine in the presence of a catalyst comprisingan oxide of aluminium and/or silicon. Subsequently, the3-methylpiperidine is passed over a dehydrogenation catalyst andconverted to 3-picoline. The 3-picoline is converted to 3-cyanopyridinewith a further catalyst. Finally, the nicotinic acid amide is obtainedin an enzymatic reaction.

In the art, various catalysts are known for dehydrogenating cyclicalkanes to arylic compounds. For instance, U.S. Pat. No. 4,401,819discloses the use of palladium deposited on silica, alumina or carbonfor the preparation of pyridine and substituted pyridines frompiperidine and related compounds.

A method for preparing 3-methylpyridine from 3-methylpiperidine with adehydrogenation catalyst is also disclosed in CN 1903842 A. In thisprocess, the catalyst is based on palladium coated on a silicon dioxidecarrier.

Specific catalysts for converting 3-methylpiperidine to 3-methylpyridineare also disclosed in WO 94/22824. The catalysts consist of palladium orplatinum as the active component coated on a carrier comprising oxidesof aluminium and/or silicon. In a specific embodiment, thedehydrogenation catalysts are obtained by impregnating silicon-aluminiumoxide with a solution of a palladium-ammonia complex.

Thus, there is a continuing need for efficient processes for producingmethylpyridines and for efficient catalysts, which are readilyavailable. Specifically, there is a need for efficient catalysts whichallow conversion of methylpiperidine to methylpyridine with a highyield. Further, there is a need for catalysts and processes, which keepthe amount of undesired side-products low.

Another problem of palladium-based catalysts is that they can easily beinactivated by oxygen or other process chemicals (catalyst poisoning).Therefore, when such catalysts are used in an industrial process, theirlifetime is limited. There is thus a need for catalysts, which arestable against inactivation and can be used in an industrial process foran extended time. Further, there is a need for processes for theproduction of methylpyridine, in which the conditions are adjusted suchthat catalysts can be used for a long time. The increase of catalystlifetime is significant for reducing the costs of such a process,because palladium is an expensive precious metal. Further, theinterruption times of the industrial continuous production process canbe reduced when the catalyst is reactive over a long time period. Thus,costs can be kept low and product uniformity is preserved.

DISCLOSURE OF THE INVENTION

Subject of the invention is a process for the production of a catalystfor the dehydrogenation of methylpiperidine to methylpyridine,comprising in the order (a) to (d) the steps of

(a) providing a carrier comprising 65-100 weight % silicon oxide and0-35 weight % aluminium oxide,(b) impregnating the carrier with palladium, whereby the carrier isbrought into contact with an aqueous solution of apalladium-ammonia-complex to obtain a catalyst,(c) drying the catalyst at a temperature below 80° C. and(d) calcinating the catalyst at a temperature below 200° C.

In a specific embodiment of the invention, the steps (a) to (d) arecarried out in one single reactor.

In a preferred embodiment of the invention, the drying step (c) iscarried out with air and/or at a temperature between 20° C. and 60° C.,preferably between 25° C. and 50° C. or between 30° C. and 45° C. In apreferred embodiment, the drying step is carried out at 40° C.Preferably, the drying step is carried out under air. The drying step isfinished when essentially all the water is removed from the catalyst. Inan embodiment of the invention, the drying step is carried out for 5hours to 7 days, preferably for 1 to 5 days.

In a preferred embodiment of the invention, the calcinating step (d) iscarried out under air and/or at a temperature between 80° C. and 200°C., preferably between 100° C. and 180° C., more preferably between 100°C. and 160° C., even more preferably between 120 and 140° C. In apreferred embodiment, the calcinating step is carried out at 130 or 140°C. The calcinating step may be carried out for a time range between 2and 72 hours, more preferably between 6 and 36 hours. In a preferredembodiment, the calcinating step (d) is carried out for 8 h at 140° C.In general, if the temperature of the calcination step is set relativelyhigh, a lower treatment time is necessary and vice verse.

In the drying step (c) water is removed from the catalyst. In this step,essentially water is removed which is not crystal water, but merelywettening the catalyst due to the aqueous production process. In thecalcinating step (d) the crystal structure of the catalyst is amended.In this step, crystal water may be removed from the catalyst. Of course,also residual non-crystal water may be removed in this step.Surprisingly, it was found that a highly efficient catalyst can beobtained when carrying out a drying step (c) and a calcinating step (d)at relatively low temperatures as outlined above. It was found that whencalcinating the catalyst at higher temperatures, the efficiency of thecatalyst is strongly decreased. Further, it was found that whenincluding a drying step at a low temperature as outlined above, theefficiency of the catalyst is significantly increased. Altogether, thesefindings were surprising because in the art drying and calcinating wereusually combined in one step, or a calcinating step was applied at asignificantly higher temperature. For instance, CN 1903842 disclosescalcination of a catalyst at 650° C., combined with drying at 110° C. to120° C.

In a preferred embodiment of the invention, after the drying step (d)the catalyst is activated in a step (e) with hydrogen. It was found thatthe catalytic activity of the catalyst of the invention is significantlyenhanced upon activation. It is not necessary to activate the catalystdirectly after drying and calcinating. In contrast, the catalystobtained after the calcinating step (d) was found to be relativelystable after calcinating and may be stored or transported. Preferably,the activation of the catalyst (e) is carried out immediately beforeusing the catalyst in the dehydrogenation process. Preferably, the timebetween activation and use of the catalyst is shorter than one hour,preferably less than 10 or 30 minutes. After activation, the catalystwas found to be labile. Preferably, it should be continuously besubjected to a hydrogen stream between activation and use. In anembodiment of the invention, the activation (e) is carried out in thesame reactor in which the subsequent dehydrogenation reaction is carriedout.

The activating step (e) is carried out under a hydrogen stream. In afurther embodiment of the invention, the activating step (e) is carriedout under hydrogen and nitrogen. For instance, the mixture may comprise20 to 80% hydrogen and 20 to 80% nitrogen, preferably 50% hydrogen and50% nitrogen (volume/volume). In a preferred embodiment, the activatingstep is carried out at least in part at an elevated temperature. It ispreferred that after the calcination step, the catalyst is cooled orallowed to cool, preferably to room temperature or to a temperaturebelow 40° C. The initial activation with hydrogen may start at thistemperature. For instance, the activation temperature may be between 25°C. and 450° C. In a preferred embodiment, the temperature is increasedduring the activating process, for example up 350° C. or up to 300° C.Whilst increasing the temperature, an amount of hydrogen may be addedwhich is adapted to the rising temperature. In a preferred embodiment,the temperature is increased until the reaction temperature of thesubsequent dehydrogenation reaction is reached. In a preferredembodiment, the temperature is increased to between 250 and 320° C.,preferably to about 290° C., and the subsequent reaction is carried outat that temperature.

The activating step (e) is carried out under exclusion of oxygen. In apreferred embodiment of the invention, the activating step (e) iscarried out under active depletion of oxygen. It was found that thecatalyst is more efficient when oxygen is strictly excluded from thereactor during treatment with hydrogen. In a preferred embodiment, adeoxygenation catalyst is used for actively depleting the hydrogenstream and/or the reaction vessel of oxygen. It was found that oxygendepletion can be significantly supported by a catalyst, which convertsoxygen and hydrogen to water. The deoxygenation catalyst may comprisepalladium. The palladium may be coated on a support, such as alumina. Ina specific embodiment, a conventional exhaust gas catalytic converter isused. A preferred deoxygenation catalyst is available under thetrademark PuriStar R0-25 S6 from BASF AG In a preferred embodiment ofthe invention, the deoxygenation catalyst is provided in a double shellpipe.

In step (b), the carrier is impregnated with an aqueous solution of apalladium-ammonia complex. Preferably, the solution is obtained bypreparing a solution of palladium chloride and dissolving ammonia in thesolution. Preferably, the impregnation of the carrier is carried out for6 hours to 72 hours, preferably for about 24 hours. During theimpregnation step, the carrier is preferably stirred. Alternatively, thecarrier is arranged as a fixed bed and impregnation solution flowsthrough it.

The carrier used in step (a) comprises silicon oxide and optionallyaluminium oxide. In a specific embodiment, the catalyst consists ofsilicon oxide. In a preferred embodiment, the catalyst essentiallyconsists of 65 to 100 weight % silicon oxide and 0 to 35 weight %aluminium oxide. The catalyst might comprise below 5 weight %, 1 weight%, or 0.5 weight % of other components, for instance due to impurities.Preferably, the silicon oxide is SiO₂ and the aluminium oxide is Al₂O₃.For instance, the catalyst is obtainable by preparing a mixed oxide ofAl₂O₃ and SiO₂. Preferably, the catalyst is prepared in a sol/gelprocess. Such carrier materials are known in the art and commerciallyavailable. A useful carrier based on silicon oxide and aluminium oxideis Grace Davicat E501™ from Grace Inc. However, the catalyst may alsohave a special crystallized structure, such as an aluminium silicate ora zeolite. Preferably, the specific surface area of the catalyst is atleast 50 m²/g, more preferably at least 100 m²/g. The specific surfacearea may be in the range of 100 to 700 m²/g, or between 200 to 500 m²/g,and is preferably about 300 m²/g.

The carrier is provided in the form of a granulate. The average diameterof the granules may be between 0.05 and 10 mm, preferably between 0.1and 5 mm or between 0.5 and 2 mm. In a preferred embodiment, shortstrands of the carrier are used, for instance the strands may have adiameter between 0.2 and 3 mm, or between 0.5 and 1.5 mm, and a lengthof 2 to 10 mm, preferably 4 to 8 mm. Prior to the treatment with thepalladium-ammonia complex, the carrier may be dehydrated. The carrier isa Lewis acid by nature. Therefore, in the inventive process the carrieris preferably neutralized with ammonia before the impregnating step (b).When a non-porous carrier is used, the palladium is attached to thesurface of the carrier. In a preferred embodiment of the invention, thecatalyst comprises 0.5 to 8 weight %, preferably 1 to 6 or 2 to 5 weight%, palladium.

Another subject of the invention is a dehydrogenation catalyst for theconversion of methylpiperidine to methylpyridine, obtainable by aprocess of the invention. The catalyst of the invention is a solidcatalyst. The catalyst comprises a silicon/aluminium core and is coveredwith an outer layer comprising palladium.

Subject of the invention is also a process for the production ofmethylpyridine from methylpiperidine, wherein methylpiperidine iscontacted with a dehydrogenation catalyst of the invention.

Methylpyridine is also referred to as picoline. The methylpyridine ofthe invention can either be 2-, 3- or 4-methylpyridine. In line withthis, the corresponding methylpiperidine can be 2-, 3- or4-methylpiperidine. In a preferred embodiment of the invention, themethylpiperidine is 3-methylpiperidine. In this embodiment,3-methylpiperidine is dehydrogenated to obtain 3-methylpyridine.

In a preferred embodiment of the invention, the reaction is carried outunder a hydrogen and/or nitrogen atmosphere.

In a preferred embodiment of the invention, the reaction is carried outin the gaseous phase at a temperature between 180° C. and 400° C., morepreferably between 200° C. and 350° C. or 200° C. and 300° C. At thesetemperatures, the educt and the product are gaseous. Of course, thecatalyst remains in a solid state. It is preferred that themethylpiperidine is passed through a reaction zone, in which it iscontacted with the catalyst. For example, the catalyst is in a containerwith an inlet and an outlet, such that the methylpiperidine is fed intothe inlet and the product is removed through the outlet. Preferably, thecontainer is a tube, a tube bundle, a pipe or a vessel.

In a preferred embodiment of the invention, the catalyst is mixed withaluminium. Surprisingly, it was found that the catalyst of the inventionis so highly reactive that it can be “diluted” with aluminium whilstpreserving a high catalytic efficiency. The addition of aluminium isadvantageous, because a palladium-based catalyst is expensive and thusthe costs can be reduced significantly. For instance, the catalyst canbe admixed with 1 to 90 weight %, preferably with 10 to 80 weight %aluminium. It was found that the catalyst is still highly active whentwo thirds of aluminium granulate are added. It was found that theactivity can be further increased when the aluminium is defatted priorto use. The reaction from methylpiperidine to picoline is endothermic,which means that thermal energy has to be supplied to the reaction zone.The addition of aluminium is advantageous, because the aluminiumsupports the heat transport to the strongly endothermic reaction.

In a preferred embodiment of the invention, the methylpiperidine isinitially contacted with a first catalyst/aluminium mixture, andsubsequently contacted with a second catalyst/aluminium mixture, whereinthe ratio of catalyst/aluminium in the first mixture is lower than inthe second mixture. This means, that initially the educt is brought intocontact with a reaction zone having a relatively high aluminium content,whereas subsequently the reaction mixture, which already comprises theproduct at least in part, is brought into contact with a reaction zonehaving a relatively low aluminium content. It was found that it isadvantageous when in the inventive process the methylpiperidine is firstcontacted with a catalyst mixture with a high aluminium content.Thereby, the good heat transfer properties of the aluminium areexploited. When emerging deeper into the reaction zone, the turnover isdecreasing since the amount of educt is decreasing. At this stage, it isadvantageous to use a catalyst mixture with a relatively high palladiumcontent. In one embodiment, a catalyst with an increasing gradient ofpalladium/aluminium can be used in the reaction. In another embodiment,two or more reaction zones comprising mixtures of catalyst withaluminium at different ratios can be employed.

In a preferred embodiment of the invention, the methylpiperidine isproduced in a cyclization reaction from methyl-1,5-diaminopentane beforethe dehydrogenation reaction. The cyclization reaction is carried out ina first reactor and the dehydrogenation reaction is carried out in asecond reactor. Both reactors are interconnected and both reactions arecarried out in a continuous process. In such an embodiment, theconversion of methyl-1,5-diaminopentane to 3-methylpyridine can becarried out in one continuous process. Such continuous processes areknown in the art and for instance disclosed in WO 94/22824 and EP 0 770687 B1.

In one embodiment, the cyclization catalyst is supplied directly on topof the dehydrogenation catalyst and the methyl-1,5-diaminopentane issupplied from above. However, it is preferred that both catalysts areprovided in distinct reactors, which are connected. In this arrangement,the temperatures and the catalysts can be controlled independently. Inthis embodiment, additional means, such as a condenser or distiller, maybe positioned between the two reactors for removing organic substanceswhich have high boiling points. Otherwise, such substances might impactactivity and lifetime of the dehydrogenation catalyst.

The methylpiperidine produced in the cyclization reaction is obtained ina mixture with ammonia. In the cyclization reaction, one equivalentammonia is obtained for each equivalent methylpiperidine. In a preferredembodiment of the invention, the mixture is fed into the second reactorwithout prior separation of the ammonia. This is advantageous because noremoval of ammonia is necessary and the process is considerablysimplified.

After the dehydrogenation reaction, the product may be isolated.Alternatively, the product may be directly introduced into a thirdreactor and subjected to a subsequent synthesis step. The remaininggaseous mixture, optionally after washing out the ammonia, comprisesHydrogen. It may be mixed with air and burned, and the energy thusgained may be retransferred into the process. In an embodiment of theinvention, after separation of the product and at least other highmolecular weight components and side products, a remaining gaseousmixture is reintroduced into the first reactor.

In a specific embodiment, the reactions are carried out in a nitrogen,hydrogen and ammonia atmosphere. It is preferred that no air or oxygenis added or present, except low impurities. For instance, the gaseousatmosphere comprises more than 50 vol. %, more than 70 vol. % or morethan 90 vol. % hydrogen. High levels of hydrogen of above 90 vol. % areapplied if an ammonia washer is used in the process. Otherwise, lowerlevels of about 70 vol. % hydrogen are preferred.

It was found that the inventive catalysts are highly efficient in theconversion of methylpiperidine to methylpyridine. On the one hand, ahigh yield of methylpyridine is obtained. Preferably, the yield ofmethylpyridine is above 90, 95 or 97%. In addition, the catalyst wasfound to be highly stable. It was found that the catalyst can be used atleast 300 days when used at temperatures between 285 and 310° C. Thelifetime of the catalyst depends on the throughput, and can be enhancedif the throughput is reduced. At lower throughputs, the catalysts couldbe used for more than 580 days. Normally, the catalyst can be used forat least one year, provided that no air enters the process and nocatalyst poisons are present. The high efficiency of the catalyst isobtained as a result of the inventive production method with thespecific drying, calcinating and activating steps. The catalyst is alsohighly efficient, if ammonia is present in the reaction process in highamounts. Due to the high activity, the catalyst can be admixed withaluminium, which saves costs and supports the endothermic reaction.

EXAMPLES 1. Production of a Dehydrogenation Catalyst

A tubular reactor (3 m long, 25 cm diameter) is filled with 75 kgcarrier (particles of silica and alumina, Davicat E501™ from Grace Inc.)and ammonia gas (5 kg) is passed above the solid catalyst at roomtemperature, leading to a neutralisation of acidic surface parts of thecarrier and its warm-up to about 60° C. At the same time, a palladiumsolution is prepared by dissolving 9.5 kg PdCl₂ and 14 kg ammonia in1800 l water. After cooling of the carrier and the palladium solution toroom temperature, said palladium solution is pumped over the carrier for16-24 h. Afterwards, the remaining solution is eluted from the reactorand the catalyst is washed twice with water. The catalyst is dried at40° C. with air, until most of the water is removed from the catalyst.After drying, the catalyst is calcinated with air at about 130° C. forabout 8 hours.

It was found that the catalyst thus obtained can be used in the processfor the production of 3-methylpyridine from 3-methylpiperidine at leastfor 300 days at temperatures between 285 and 310° C. The lifetime of thecatalyst depends on the throughput, and can be enhanced if thethroughput is reduced. At lower throughputs, the catalysts could be usedfor more than 580 days.

2. 3-Picolin Production with the Catalyst According to the Invention

300 g catalyst was filled into a reactor vessel. The catalyst wasactivated with a gaseous mixture of 50% hydrogen and 50% nitrogen(volume/volume) having an initial temperature of 50° C., which wasincreased to 300° C. Subsequently, a gaseous mixture ofmethylpiperidine, hydrogen and nitrogen having a temperature of 300° c.was passed over the catalyst. With feed stream of 0.9 kg/hmethylpiperidine, 0.3 kg/h nitrogen and 3 g/h hydrogen, the followingresults were obtained (Table 1):

TABLE 1 Reaction Time [h] 3-Picolin yield [%]  0:21 96.69  1:21 97.07 12:41 98.30  40:41 98.49  77:06 98.40 100:51 98.10 169:11 98.55 194:1998.43 256:33 98.32 329:14 98.43 407:21 98.42 449:51 98.39 476:31 98.35509:56 98.42

As can be seen, the catalyst according to the invention allows for theproduction of 3-picolin with a high yield which remains constant overlong periods of time.

1. A process for the production of a catalyst for the dehydrogenation ofmethylpiperidine to methylpyridine comprising in the order (a) to (d)the steps of (a) providing a carrier comprising 65-100 weight % siliconoxide and 0-35 weight % aluminium oxide, (b) impregnating the carrierwith palladium, whereby the carrier is brought into contact with anaqueous solution of a palladium-ammonia-complex to obtain a catalyst,(c) drying the catalyst with air at a temperature below 80° C. and (d)calcinating the catalyst at a temperature below 200° C.
 2. The processof claim 1, further comprising after step (d) (e) activating thecatalyst with hydrogen.
 3. The process of claim 1, wherein the dryingstep (c) is carried out at a temperature between 20° C. and 60° C. 4.The process of claim 1, wherein the calcinating step (d) is carried outwith air and/or at a temperature between 80° C. and 200° C.
 5. Theprocess of claim 1, wherein the activating step (e) is carried out underactive depletion of oxygen.
 6. The process of claim 1, wherein thecatalyst comprises 0.5 to 8 weight % palladium.
 7. A dehydrogenationcatalyst for the conversion of methylpiperidine to methylpyridine,obtainable by a process of claim
 1. 8. A process for the production ofmethylpyridine from methylpiperidine, wherein methylpiperidine iscontacted with a dehydrogenation catalyst according to claim
 7. 9. Theprocess of claim 8, wherein the methylpiperidine is 3-methylpiperidine.10. The process of claim 8, wherein the reaction is carried out under ahydrogen and/or nitrogen atmosphere.
 11. The process of claim 8, whereinthe reaction is carried out in the gaseous phase at a temperaturebetween 180° C. and 400° C.
 12. The process of claim 8, wherein thecatalyst is mixed with aluminium.
 13. The process of claim 12, whereinthe methylpiperidine is initially contacted with a firstcatalyst/aluminium mixture, and subsequently contacted with a secondcatalyst/aluminium mixture, wherein the ration catalyst/aluminium in thefirst mixture is lower than in the second mixture.
 14. The process ofclaim 8, wherein before the dehydrogenation reaction themethylpiperidine is produced in a cyclization reaction frommethyl-1,5-diaminopentane, wherein the cyclization reaction is carriedout in a first reactor and the dehydrogenation reaction is carried outin a second reactor, both reactors are interconnected and both reactionsare carried out in a continuous process.
 15. The process of claim 8,wherein the methylpiperidine produced in the cyclization reaction isobtained in a mixture with ammonia, and the mixture is fed into thesecond reactor without prior separation of the ammonia.